1. Field of the Invention
The present invention relates to a storage device, and technology relating to its control and manufacture, and more particularly, to an effective technology utilized in the storage unit, such as a magnetic disk device that utilizes magnetic disks as storage media.
2. Description of the Related Art
Similarly to examples described in patent literature 1, 2, and 3, it is widely known that a magnetic disk device comprises a load and unload system that withdraws the magnetic head when device operation stops, so that the magnetic head floating over the magnetic disk does not touch the magnetic disk, and moves the magnetic head back onto the magnetic disk from the position where the head was withdrawn when device operation is restarted.
Specifically, a sloped unit called a ramp is located adjacent to the periphery of the magnetic disk, and when rotation of the magnetic disk is ceased, the magnetic head is withdrawn onto the ramp on the unload operation initiated. When the magnetic disk is rotated and after the spinning speed of the magnetic disk has reached a certain speed, the magnetic head sitting on the ramp is moved to the magnetic disk, that is, the load operation is carried out.
This system securely prevents contact between the magnetic head and the magnetic disk when the operation of the magnetic disk ceases, and consequently prevents problems such as starting errors and damage to the magnetic disk surface where the data is stored, which are both caused by the adhesion of the magnetic head to the surface of the magnetic disk.
Further, this load and unload system can be used to resist shock, and is used for magnetic disk devices in laptop computers, which are required to be highly shock-resistant.
When this load and unload operation system is adopted, even in the event of sudden power failure, the unload operation ( emergency unload operation) can still be completed without the power supply by inputting the spindle motor back electromotive force, obtained from the spindle motor connected to the magnetic disk spinning under its own inertia, to the load and unload operation system, which consists of the voice coil motor.
However, the energy, required to overcome the friction with the ramp and to operate the unloading (hereafter called unloading energy), tends to increase in proportion to the number of unload operations. This characteristic of the unloading energy causes a technical issue, when the unloading energy becomes greater than the spindle motor back electromotive force, the emergency unload cannot be performed in power failure, and a starting error occurs when the device is restarted because the magnetic head and the magnetic disk are left in contact.
That is, in restarting the magnetic disk spin by the spindle motor, a starting error occurs because of the friction and adhesion between the magnetic head and the magnetic disk, leading to the serious error that the data cannot be read from the magnetic disk device.
The patent literature 1 discloses technology that controls the loading and unloading using a voice coil motor by detecting the speed from the voice coil motor back electromotive force that drives the head. However, the above-explained technical issue caused by the change in the unloading energy is not mentioned.
Also the patent literature 2 discloses a technology based on the control of the speed of the load/unload preventing the collision between the head and the disk during loading by the feedback of the voice coil motor back electromotive force to the driving circuit of the voice coil motor. However, the technical issue caused by the change in the unloading energy is again not recognized.
Moreover, during the regular unload operation, when the unloading fails without confirming whether the head is parked or not, the spin of the disk ceases with the head remaining on the magnetic disk. Therefore the technical issue still remains unsolved, and upon restarting the device a starting error occurs for the same reason as explained above.
For this reason, patent literature 3 discloses technology that prevents unloading failures by detecting the moving speed of the head slider during unloading, and by determining whether the head slider reached a predetermined unloading position or not based on the detected moving speed. However, because detection of the unloading completion is based on the moving speed of the head slider and it is an indirect approach, there remains concern about the low accuracy of the method.    Patent literature 1: Japanese Publication Unexamined Application No. 2001-155455    Patent literature 2: Japanese Publication Unexamined Application No. Heisei 8-63920    Patent literature 3: Japanese Publication Unexamined Application No. Heisei 11-16136